1. Field of the Invention
The present invention relates to a radio terminal, a relay station, a radio base station, and a communication method among these stations. The present invention is particularly suitable for use on the occasion of addition of a relay station on the basis of a radio communication system specified, for example, by IEEE802.16.
2. Description of the Related Art
A radio communication system to realize communication via a radio communication path is now spreading in the world-wide scale as a representative system of the WCDMA and CDMA 2000 or the like. In this radio communication system, a plurality of radio base stations are provided for service areas and a radio terminal makes communication with the other communication devices (communication terminals) via any of the radio base stations. Moreover, in such system, a service area formed by a base station is overlapped with another service area formed by an adjacent base station. Therefore the communication can be maintained by a handover process even though radio environment of the communication gets worse.
Moreover, as a radio communication system, the technologies, for example, code division multiplex, time division multiplex, frequency multiplex and OFDM (OFDMA) are generally employed and thereby a plurality of radio terminals can be connected to a radio base station simultaneously.
However, when a radio terminal is close to the boundary of a service area formed by a base station even within the service area, high-speed communication becomes difficult for the radio terminal because the radio communication environment is deteriorated.
Therefore, a communication system has been proposed, in which a relay station is provided within the service area of a radio base station to realize radio communications between a radio terminal and the radio base station via the relay station.
Particularly, introduction of such relay station (RS) is now discussed by the task group of 802.16j.
Matters regarding IEEE802.16 are disclosed, for example, in the IEEE Std802.16™-2004 and the IEEE Std802.16e™-2005.